1. Field of the Invention
The present invention relates to a fast handoff method with (CoA) Care-of Address pre-reservation and routing in use of an access point in a wireless network, and more particularly to a fast handoff method using an access point capable of receiving a Router Advertisement (RA) message including a pre-reserved (CoA) Care-of Address by the access point reducing handoff time when a mobile terminal moves from one network to another network.
2. Description of the Related Art
The number of wireless Internet users has increased rapidly recently, together with widespread Internet use, fast development in wireless communication technologies and mobile terminals such as portable computers and Personal Digital Assistants (PDAs). The mobile terminals under the wireless Internet environment move anytime to different places changing network access positions. Such mobile terminals are referred to as mobile nodes.
In order for a mobile node to perform wireless Internet communications, Internet services should be secured in as much a high quality as a home network even when the mobile node moves to a foreign network beyond its own home network. Diverse technologies have been proposed to provide stable wireless Internet services even when the mobile node changes network access positions.
In particular, the Internet Engineering Task Force (IETF) mobile IP working group has proposed a method in which all mobile terminals continuously use a specific identifier referred to as ‘IP address’ regardless of network access positions, and continues work for defining a protocol for mobile IP and complementing a drawback to the protocol. Furthermore, in order to solve a problem that the existing IPv4 system becomes insufficient to accommodate increasing address demands, the mobile lPv6 technology is being introduced to provide wireless Internet services in use of the IPv6 protocol.
According to the mobile IPv6 technology, a mobile node communicates with a correspondent node (CN) in use of its own home address (HA) through a home agent which is a router having its own registration information even though it moves to a foreign network. To do this, if the mobile node is linked to a foreign network, the mobile node is assigned a CoA which is a temporary address from an access router of the foreign network, and registers the assigned CoA to the home agent with a home address.
Therefore, as the mobile node moves to a new network, three operations are taken for mobile node movement detection: CoA configuration, duplicate address detection (DAD), and the binding process by sending to a home agent and a correspondent node a binding update message including the CoA. These operations cause handoff delays and packet losses and, in particular, degrade the quality of services (QoS) requiring real-time transmissions.
In the meantime, in order for an Access Point (AP) to provide its services under the wireless LAN environment, a mobile node has to be in its service-available range, and, if the mobile node is out of the service-available range, the mobile node has to receive the services from a new Access Point (AP). As above, if a mobile node moves from one network to a different network, the aforementioned three steps for the handoff are required.
FIG. 1 shows a mobile node belonging to one network moving to a different network under the general wireless LAN environment.
According to the mobile lPv6, if a mobile node (MN) belonging to a network A moves to a network B which is controlled by a new Access Router, the mobile node has to establish a layer-2 association. That is, the mobile node needs to complete the link-layer attachment, receive a Router Advertisement (RA) message from a new Access Router during a process for layer-3 handoff, and perform movement detection.
Subsequently, the mobile node generates a new CoA using network prefix information included in an RA message and its own Link-Layer Address (LLA) and performs Duplicate Address Detection (DAD).
FIG. 2 is a flow chart showing a process of receiving an RA message from a new Access Router, configuring a CoA, and performing the Duplicate Address Detection, when a mobile node belonging to one network moves to a different network in the prior art.
If a mobile node moves to a different network (S210), the mobile node sends an Association Request message to an Access Point (AP) for a link to the Access Point, and receives an Association Response message when services can be supported by the Access Point, to thereby complete Link-Layer attachment (S215).
After the mobile node completes the Link-Layer attachment, the mobile node waits for an RA message to be received from an Access Router (AR) of the network (S220). For this, the mobile node can multicast a Router Solicitation (RS) message to the whole network.
The Access Router periodically receives the RS message, for example, every three seconds, and sends the RA message (S225). Thus, at worst, the mobile node can miss its movement for three seconds.
The RA message provides network prefix information. Therefore, a mobile node obtains a new network prefix from the received RA message, and is notified of its own Access Router. The mobile node generates a new CoA using the network prefix information and its own Link-Layer Address (S230).
Next, the mobile node multicasts to its newly linked network a Neighbor Solicitation (NS) message including the generated CoA and its own Link-Layer address, and starts performing the Duplicate Address Detection (S235).
The Duplicate Address Detection becomes successful when the mobile node does not receive the Neighbor Advertisement (NA) notifying of address duplication in a predetermined limited time, and the corresponding CoA is recognized as a unique CoA, so the mobile node performs communications in use of the CoA (S235). The predetermined limited time is 1000 ms by default.
However, if there is a node using the corresponding CoA in advance, the node sends to the mobile node the Neighbor Advertisement including its own Link-Layer address in response to the Neighbor Solicitation message (the DAD failed). Accordingly, the mobile node randomly generates an ID for a corresponding interface to obtain a new CoA (S230), and performs the Duplicate Address Detection again for the generated CoA (S235). If a unique CoA cannot be obtained after the Duplicate Address Detection is consecutively performed five times, the mobile node gives up communications under the corresponding foreign network and waits until a link is changed.
As described above, according to the conventional mobile IPv6 protocol, a mobile node has to wait a certain time to receive a Router Advertisement message and recognize a router to which the mobile node belongs, when the mobile node is to have access to a new Access Router. Further, undesired delays occur since the mobile node has to wait for a response message even when the mobile node immediately sends a Router Solicitation message.
Since the mobile node generates a CoA and performs Duplicate Address Detection after receiving a Router Advertisement message, some time is spent when the Duplicate Address Detection is successfully completed, and more time is spent when the Duplicate Address Detection fails.
Accordingly, a considerable packet loss occurs since the mobile node cannot perform normal communications during the time which the handoff is delayed. In particular, when the mobile node sends real-time data under the mobile IP environment, the data loss occurring during such a delay time causes a serious problem securing the quality of services.
Therefore, efforts have been made to reduce the time it takes to receive a Router Advertisement and the DAD performance time.